As a subclass of commercially available polymers, polyurethane elastomers have several properties whose advantages confer unique benefits on these products. Typically, polyurethanes show high abrasion resistance with high load bearing, excellent cut and tear resistance, high hardness, resistance to ozone degradation, yet are pourable and castable. Compared to metals, polyurethanes are lighter in weight, less noisy in use, show better wear and excellent corrosion resistance while being capable of cheap fabrication. Compared to other plastics, polyurethanes are non-brittle, much more resistant to abrasion, and exhibit good elastomeric memory. Polyurethanes can be used for coatings and adhesives, utilizing secondary amine curing agents, such as taught in applicant's prior U.S. Pat Nos. 4,578,446 and 4,714,512. Polyurea can also be used as disclosed in applicant's prior U.S. Pat. No. 4,663,201 and Dominguez U.S. Pat. No. 4,513,133. Among well-known catalysts used for controlling competing reactions are tertiary amines, organometallic compounds, alkali metal salts of carboxylic acids and carboxylic acids. Also, it may be considered known to cure polyurethane prepolymers with a curing system comprising a dialkyl aromatic secondary amine, a polyol and a primary amine at room temperature using a catalyst system comprising adipic acid and a tin catalyst. However, some applications, such as coatings for concrete structures, such as roads, bridge abutments, parking lots, etc., must also have a very low moisture sensitivity and organo tin catalysts, such as dibutyl tin dilaurate, catalyze reaction of the prepolymer with water and, therefore, can only be used in small amounts, and care must be taken to keep exposure to moisture at a minimum.
Part of the utility of polyurethanes derives from their enormous diversity of properties resulting from a relatively limited number of reactants. Typically, polyurethanes are prepared on site by curing urethane prepolymers, which are adducts of polyisocyanates and polyhydric compounds. A large class of such prepolymers are approximately 2:1 adducts of a diisocyanate, OCN--Y--NCO, and a diol, HO--Z--OH, whose resulting structure is OCN--Y--NHCO.sub.2 --Z--OCONH--Y--NCO. Although Y is susceptible of great variety, usually being a divalent alkyl, cyclohexyl, or aromatic radical, in fact the most available urethane prepolymers are made from 2,4-toluenediisocyanate (TDI), or 80/20 mixtures with 2,6-toluenediisocyanate or 4,4'-methylenediphenyldiisocyanate (MDI). The diols forming the "backbone" of the polymer, containing the "soft segments," display a greater range of variety; Z may be a divalent alkyl radical (i.e., an alkylene group) and frequently are ethers or esters which are condensation products of glycols with alkylene oxides and dicarboxylic acids, respectively.
Polyureas are prepared in similar, known manner as the polyurethane prepolymers described above except that the backbone of the polymer is formed by the reaction of a polyamine (rather than a polyol) with a diisocyanate. The polyamines and polyols used in the reaction will be referred to as "backbone" polyols or "backbone" polyamines to distinguish them from the curing agents of the present invention.
In the so-called "one-shot" process, a separate step of forming a prepolymer is eliminated and all reactants are brought together at the same time or substantially simultaneously. This term may also be applied where the typical "prepolymer" components are brought together first and within a very short time the curing agent and other additives are mixed together. The "one-shot" method of processing is particularly prevalent in MDI- or modified MDI-based systems. The process generally requires that the various components have similar reactivities with the isocyanate components. The higher heat of reaction creates limitations and some complications with larger cast parts of thicker coatings, but is not particularly deleterious or can be tolerated in the applications contemplated here, and, in fact, may be advantageous in promoting a faster cure without requiring the addition of heat.
"Quasi-prepolymers" may also be formed, in which only a part of the theoretical equivalents of the backbone polyol are reacted with the isocyanate. The remaining backbone polyol is added at the time the curing agent and catalysts are blended.
Polyurethanes and polyureas are formed by curing the urethane prepolymer. Curing is the reaction of the terminal isocyanate groups of the prepolymer with active hydrogens of a polyfunctional compound so as to form high polymers through chain extension and, in some cases, cross-linking. Diols, especially alkylene diols, are the most common curing agents, especially for MDI-based urethane prepolymers, and representing such diols with the structure HO--X--OH, where X is an organic moiety, most usually an alkylene group, the resulting polymer has as its repeating unit, EQU --Y--NHCO.sub.2 --Z--OCONH--Y--NHCO.sub.2 --X--OCONH--
Where a triol or a higher polyhydric alcohol is used, cross-linking occurs to afford a nonlinear polymer.
Other polyfunctional chemicals, especially diamines, are suitable as a curing agent. For example, 4,4'-methylene-bis-orthochloroaniline, usually referred to as MOCA or MBOCA, is a primary diamine curing agent which is both a chain extender and a cross-linker for TDI-based urethane prepolymers. Generally speaking, however, primary diamines react with prepolymers, and especially MDI-based prepolymers, so quickly that they are not usable as curing agents. Recently, certain secondary diamines have been found to have an acceptably long pot life, and act as chain extenders with urethane prepolymers. Such secondary diamines as N,N'-dialkyl-4,4'-methylenedianilines, N,N'-dialkyl-phenylene-diamines, and polyfunctional oligomers based thereon, are generally effective curing agents for a broad range of urethane prepolymers at elevated temperatures. Polyhydric alcohols have also been used as curing agents because their reaction with urethane prepolymers is sufficiently fast to be convenient, but not so fast as to make it difficult to work with the resulting polymer.
Previous attempts to cure polyurethane and polyurea coatings at ambient temperature have involved the use of a curing agent which includes a primary amine which, as mentioned above, are very fast. To applicants' knowledge, no one has utilized our secondary amine curing agents under ambient conditions to obtain an adequately cured TDI or MDI-based polyurethane coatings without using a prmary amine as a co-curing agent.
Polyurethanes find extensive application as coatings and adhesives. Among the properties of polyurethanes particularly desirable in the coating art are their chemical resistance, light stability, flexibility, toughness, weatherability, moisture resistance, abrasion resistance, gloss and color retention, and impact resistance. In polymers useful for coating or adhesive applications, it is desirable that the tack-free time be reasonably short, i.e., within about 48 hours or preferably within about 18 hours, and gel time long enough for the material to be coated onto a substrate. We have found that both TDI- and MDI-based prepolymers can be cured at ambient temperatures with the secondary amines we previously described without the inclusion of primary amine co-curing agents to obtain resins which are particularly suitable as coatings having reduced moisture sensitivity by providing a particular catalyst combination comprising an aliphatic acid, especially fatty acids, e.g., adipic, oleic or stearic acid and an organo-metallic catalytic compound, containing, especially mercury or bismuth.